The discharge of the pollutants into the atmosphere has become of increasing concern as the rate of discharge increases and as the level of pollutants in the atmosphere increases. One of the sources of air pollution is the sulfur oxides and particulate matter which are discharged with the flue gases during the combustion of sulfur-containing fuels. A number of systems have been developed or proposed to remove the sulfur oxides and particulate matter from the flue gases. One of the first commercial systems involves scrubbing the flue gases with an aqueous medium which contains materials which will react with the sulfur oxides. The most commonly used additive materials for forming the aqueous scrubbing medium are limestone, dolomite, and slaked lime. A slurry of the additive material enters directly into a reaction tank and the scrubbing medium is then conveyed from the tank to spray nozzles in the scrubber. The incoming gas, laden with dust and SO.sub.2 contacts the sprayed scrubbing medium in the scrubber. The SO.sub.2 reacts with the additive to form sulfite. If extensive oxidation of the reactants takes place, the sulfite will be converted to sulfate which can cause serious scaling problems in the scrubber and associated piping and equipment. This is related to the tendency of this material to form extensively stable supersaturated solutions. Crystallization of the sulfate as gypsum, CaSO.sub.4.2H.sub.2 0, can occur on nucleation sites on the internal surfaces of the scrubber resulting in scale formation. One system for preventing this crystallization by nucleation and the resulting scaling is to provide sufficient seed crystals of the sulfate whereby the crystallization will take place by growth of the existing crystals. However, in order to minimize the chances to sulfate scaling, it is desirable to minimize the amount of oxidation of sulfite to sulfate.